Statistical Study of Heavy Ion Outflows From Mars Observed in the Martian-Induced Magnetotail by MAVEN

It is important to include the effects of cold ions when we consider heavy ion outflows from Mars. We here report on statistical properties of heavy ion outflows (including cold ion outflows) observed by Mars Atmosphere and Volatile EvolutioN in the optical wake region. Using data from July 2015 to December 2017, we statistically investigate the effects of solar wind convection electric fields and crustal magnetic fields on the heavy ion outflows. Results show that the average density ratio of O⁺:O₂ ⁺:CO₂ ⁺ is ~29:68:04. In the southern hemisphere where the strong crustal magnetic fields are located, the heavy ion outflow flux becomes smaller and the relative contribution of molecular ions to heavy ion outflows is larger than the northern hemisphere. The solar wind convection electric field strongly affects the heavy ion outflows. Heavy ion density is larger in the −E (electric field) hemisphere than in the +E hemisphere, while the dependence of velocity is opposite. Acceleration by the solar wind convection electric field in the +E hemisphere is expected to cause these dependencies. The heavy ion flux is larger in the −E (total O: 8.6 × 10⁶ cm^–2/s) than in the +E hemisphere (2.9 × 10⁶ cm^–2/s) due to the large density in the −E hemisphere. Velocity ratios of O⁺ to O₂ ⁺ suggest that heavy ion outflows with large velocities tend to have the same energy to each other, while the O⁺ to O₂ ⁺ ions are more likely to have the same velocity in outflows with small velocities.